Petra Diamonds has increased its interest in South Africa's Koffiefontein diamond mine from 70% to 74%, news and investment outlets report.

The diamond company acquired the additional 4% interest from Re-Teng Diamonds Ltd shareholders, a company that partners with Petra on black economic empowerment at Koffiefontein.

Koffiefontein, which Petra bought from De Beers in 2007, produced 53,026 carats of rough diamonds from January-June 2010, with an average per-carat price for the period of $402.

The Koffiefontein mine was originally opened in 1870 and has periodically been closed and reopened. Over its lifespan, the mine has yielded 7.3 million carats, with the largest diamond recovered weighing 139 carats.

South Africa remains one of the world's largest diamond producers. Figures from the Kimberley Process 2009 Annual Global Summary put the country's diamond output at 6.14 million carats with a value of $885.5 million.